Broadband access networks have become the final frontier in the evolution of the broadband communications industry. While tremendous advances over the last two decades in optical-fiber networks provide enormous bandwidth (terabits per second per fiber) between all cities, emerging telecommunications services and Internet applications remain throttled by the access networks that connect end users to core networks. Widespread deployment of cable modem and digital-subscriber line access technologies has stimulated diverse bandwidth-hungry applications, demonstrating clearly the inevitable strong growth of bandwidth. But it is now accepted widely that these access technologies, built on the coaxial cable and copper wires of the past era, must soon yield to new optical-fiber-based access systems. Hence the rapidly growing fiber-to-the-x (FTTX) industry has emerged (where x=home, office, curb, etc.) providing tens of millions of optical access lines worldwide to date. Yet while major FTTX vendors and standards organizations compete, industry leaders are calling for the definition of optical access technologies that substantially outperform current FTTX systems in multiple dimensions.
Unlike core networks, where aggregated and smoothed traffic is exchanged between hundreds of end points, a set of unique challenges makes it rather difficult to introduce new technology into access networks. Access network infrastructure must connect to hundreds of millions of end points. Each end point may be inactive for extended periods of time and then demand high capacity in an instant. The cost of each access line must be justified by the revenue associated with an individual user. Given these constraints, cost is a primary concern and technology must be amortized over extended periods of time. Carriers therefore face difficult decisions in selecting access technology that cost-effectively meets demand and provides a reasonable upgrade path for future requirements.
First-generation (1G) broadband access systems, which include digital subscriber line (DSL) and cable modems (CM), use electronic signal processing to squeeze roughly 10 Mb/s downstream (to customer) and 1 Mb/s upstream out of existing metallic access wires (twisted pair for DSL and hybrid fiber coax (HFC) for CM). Ubiquitous deployment of these technologies has driven the Internet to prominence, and nears saturation in most info-tech (IT)-progressive countries. In recognition of the limits of these wires, the industry began in the 1990s to define a second generation (2G) of optical access technology targeting bandwidths of roughly 100 Mb/s. Passive optical networks (PONS) emerged as the dominant contending approach, and several varieties have been standardized (G-PON, E-PON). Commercial viability was achieved by sharing a single fiber channel by typically 32 or 64 customers. At present several tens of millions of PON-based lines have been deployed worldwide.
Given the enormous scale of the potential market for wireline access technologies (roughly $1000 per line×200 M lines in North America alone=$200 B), and the criticality of broadband communications for society, a large body of research and commercial activity continues, focused mostly on some form of PON. PONs were proposed concurrently in July 1986 by researchers at British Telecom (BT) in “Transparent single-mode fiber optical networks,” Payne, D.; Stern, J.; Lightwave Technology, Journal of Volume 4, Issue 7, July 1986 Page(s):864-869, using WDM and, given that WDM was clearly impractical at that time, using subcarrier multiplexing to manage upstream contention on the shared fiber feeder, as in “Lightwave system using microwave subcarrier multiplexing,” by Darcie, T. E.; Dixon, M. E.; Kasper, B. L.; Burrus, C. A.; Electronics Letters Volume 22, Issue 15, Jul. 17, 1986 Page(s): 774-775, and in “Wide-band lightwave distribution system using subcarrier multiplexing,” by Darcie, T. E.; Iannone, P. P.; Kasper, B. L.; Talman, J. R.; Burrus, C. A., Jr.; Baker, T. A., Sr.; Lightwave Technology, Journal of Volume 7, Issue 6, June 1989 Page(s): 997-1005. BT then proposed using time-division multiplexing and a centrally coordinated medium-access control (MAC) protocol to manage contention, in “Passive optical local networks for telephony applications and beyond,” by Stern, J. R.; Ballance, J. W.; Faulkner, D. W.; Hornung, S.; Payne, D. B.; Oakley, K.; Electronics Letters Volume 23, Issue 24, Nov. 19, 1987 Page(s): 1255-1256. This approach prevailed ultimately, and is the basis of the many existing PON standards, as described in “An introduction to PON technologies [Topics in Optical Communications],” by Effenberger, F.; Clearly, D.; Haran, O.; Kramer, G.; Ruo Ding Li; Oron, M.; Pfeiffer, T.; Communications Magazine, IEEE Volume 45, Issue 3, March 2007 Page(s): S17-S25.
The majority of current commercial activity focuses on the deployment of 2G PON FTTX systems. An example is shown in FIG. 1a. Downstream information is broadcast from the optical line terminal (OLT) 400 in the local office (LO), after separation from upstream information in a combiner 403, through a shared fiber feeder (˜10 km) 501 to a neighborhood passive splitting node 200 (PSN) to 32-64 optical network units (ONU) 100 connected through fiber drop cables 502. Aggressive carriers prefer FTTH, where an ONU serves an individual customer. Conservative carriers place the ONU in a neighborhood, with short metallic, typically twisted pair ‘drop’ cables to each home (fiber-to-the-curb, FTTC).
Advantages of this form of PON over 1G DSL or CM systems include bandwidths of roughly 1 Gb/s shared by the 64 users, ease of fiber management in the LO, only one LO transmitter (Tx) 401 and receiver (Rx) 402 pair per PON, and no power required at the PSN. However, sharing through optical splitter 201 limits the bandwidth per user, the number of OLTs in a LO becomes an operational challenge, and for FTTC the large number of active (i.e. powered) ONUs in outside cabinets undermines that advantage of the passive network.
Continued growth of the Internet and the emergence of broadband applications like video on demand, IP-TV, peer-to-peer file sharing, and telecommuting, suggest that the bandwidth sharing used in 2G systems must be revisited. This has stimulated extensive research into third-generation (3G) systems. Industry consensus strongly favors PONs based on wavelength division multiplexing (W-PON), in which each user occupies a dedicated wavelength (rather than time slot, as in 2G) within a shared fiber feeder. With a full wavelength per user, logical point-to-point communication at multi-gigabit rates is feasible. However, W-PON faces well known challenges in cost, manageability, and scalability.
Numerous improvements have been suggested that are somewhat incremental on the basic 2G PON system, to avoid the challenges of W-PON. But the 3G research leading edge, which is gaining traction rapidly within industry, favors WDM-based PON (W-PON). Surprisingly, current proposals differ little from the first system proposed in 1986 in “Transparent single-mode fiber optical networks” Payne, D.; Stern, J.; Lightwave Technology, Journal of Volume 4, Issue 7, July 1986 Page(s):864-869, as shown in FIG. 1b. Wavelengths for each ONU Tx and Rx 101, 102 are combined in a WDM 104 in the ONU, and then combined at the passive wavelength node (PWN) 300 onto the feeder fiber 501 using an arrayed waveguide grating (AWG) 305. The inverse is implemented at OLT 400, providing full duplex connections between users and the network.
Numerous variants of this basic W-PON have been proposed. A key objective is to deploy a fiber infrastructure that is capable of supporting any future requirement, while being cost effective today. Proposals include overlays using spectral slicing to broadcast over a WDM network, as described in “Enhanced privacy in broadcast passive optical networks through the use of spectral slicing in waveguide grating routers,” by P. P. Iannone; N. J. Frigo; K. C. Reichmann; Photonics Technology Letters, IEEE Volume 9, Issue 7, July 1997 Page(s): 1044-1046. In “An evaluation of architectures incorporating wavelength division multiplexing for broad-band fiber access,” by Feldman, R. D.; Harstead, E. E.; Jiang, S.; Wood, T. H.; Zirngibl, M.; Lightwave Technology, Journal of Volume 16, Issue 9, September 1998 Page(s): 1546-1559, broadcast splitters and WDM are combined in the passive splitting node. A multistage W-PON is described in “Design and cost performance of the multistage WDM-PON access networks,” by Maier, G.; Martinelli, M.; Pattavina, A.; Salvadori, E.; Lightwave Technology, Journal of Volume 18, Issue 2, February 2000 Page(s): 125-143. An interleaved W-PON is described in “A Novel Scalable Multistage DWDM PON Architecture Using Cascaded Optical Interleavers With Increasing Periodicities Controlled in Central Offices,” by Akanbi, O.; Jianjun Yu; Ellinas, G.; Gee-Kung Chang; Conference on Optical Fiber Communication and the National Fiber Optic Engineers Conference, 2007. OFC/NFOEC 2007, 25-29 Mar. 2007 Page(s): 1-3. Hybrid time- and wavelength-division multiplexing is described in “A WDM-Ethernet hybrid passive optical network architecture,” by Jeong-Ju Yoo; Hyun-Ho Yun; Tae-Yeon Kim; Kang-Bog Lee; Mahn-Yong Park; Byoung-Whi Kim; Bong-Tae Kim; Advanced Communication Technology, 2006. ICACT 2006. The 8th International Conference Volume 3, 20-22 Feb. 2006 Page(s): 4. In “A wavelength-division multiplexed passive optical network with cost-shared components,” Frigo, N. J.; Iannone, P. P.; Magill, P. D.; Darcie, T. E.; Downs, M. M.; Desai, B. N.; Koren, U.; Koch, T. L.; Dragone, C.; Presby, H. M.; Bodeep, G. E.; Photonics Technology Letters, IEEE Volume 6, Issue 11, November 1994 Page(s): 1365-1367, the use of an optical loop back to eliminate wavelength control problems is described.
Implicit in PONs where wavelengths are shared is a medium-access control (MAC) protocol and associated scheduler. All 2G PONs use a centrally-mediated MAC protocol with reservations to organize upstream transmission. Efficient operation requires accurate determination of the transmission time to each ONU (ranging). Scheduling has been considered in a shared-wavelength W-PON, as described in “Design and performance analysis of scheduling algorithms for WDM-PON under SUCCESS-HPON architecture,” Kyeong Soo Kim; Gutierrez, D.; Fu-Tai An; Kazovsky, L. G.; Lightwave Technology, Journal of Volume 23, Issue 11, November 2005 Page(s): 3716-3731.
To summarize the state-of-the-art, W-PON is seen as a likely end state, but challenges abound in reaching tolerable cost. These challenges can be classified roughly as follows. Architectural: Cost reduction requires optimization of topological design. Examples include minimizing total fiber spans, eliminating requirements for power and environmental control at network nodes, and efficient use of components through aggregation. Technology: Reducing component cost involves continuing the evolution of a defined technology towards lower cost. Wavelength-controlled lasers, WDM components, electronic drivers, and component packaging are examples. Operational: In determining the economic viability of access infrastructure over decades of use, the cost to maintain and operate the network is a main factor. For example, these costs include those associated with installation and removal driven by customer churn. State-of-the-art W-PON proposals have failed in one or more of these dimensions hence no consensus for a compelling direction has emerged.
With this invention we seek to overcome challenges with the prior art and define new systems for a next generation of future-proof all-optical access networks. These networks would become the infrastructure supporting future local wireline residential and business telecommunications. Our focus begins with the well-known concept of a passive optical network (PON) that provides a dedicated wavelength per user. A novel hierarchical wavelength multiplexing strategy, implemented in some embodiments using planar lightwave circuits defined herein, provides two key breakthroughs. First, while each PON provides a dedicated wavelength per user, this wavelength can be shared to a flexible degree with other users on other PONs. This allows introduction of tiered wavelength services. Second, it facilitates a distributed controller that mediates bandwidth on each shared wavelength such that the local office becomes simply a passive node, providing substantial operational advantages.